The well-known method of adjusting the magnetic moment of a small bar magnet to a predetermined stable value involves subjecting the magnet to a DC field of sufficient strength to cause saturation. The magnet is then removed from the DC field and is placed in an AC field of slowly increasing peak amplitude, thereby reducing the remanent magnetization. The repeated cycling caused by the AC field insures the attainment of a stable operating point free from minor hysteresis loop effects which would result in irreversible changes with temperature and/or the presence of small external fields.
Unfortunately, the uniformity of ordinary magnetic material is insufficient to permit use of the same peak AC amplitude for a number of magnets being set to a specified moment. Therefore, it is necessary to raise the AC amplitude to a trial value and then remove the degaussing field and measure the magnetic moment. By repeated trials with gradually increasing AC amplitude, the desired moment is obtained. If the moment is inadvertently reduced below its desired value, the magnet must be resaturated and the procedure repeated.
Apparatus to implement the foregoing method includes a magnetizing/degaussing solenoid or yoke and a sensing magnetometer. For convenience, the mangetometer may be mounted to sense the moment of the magnet under adjustment without physically having to remove the magnet to a test station. However, it is generally not possible to sense the moment while the degaussing field is being applied. As noted hereinbefore, it is necessary to alternately apply the AC generated degaussing field, remove the latter, and then read the magnetic moment. The procedure is repeated as many times as necessary to achieve the desired moment of strength. It is apparent that the process is laborious and time consuming. Several complete cycles, each starting with resaturation of the magnet may be required, particularly when the desired value of moment is to be kept within strict tolerances. Moreover, the operation becomes more critical when the moment is on the order of fifty-percent of saturation, since the magnetization curve is steep in this region and small AC field changes result in large moment changes. It is almost impossible to avoid overshooting the target moment value, so that recycling from saturation is often necessary.